Laminated dielectric filter

Abstract: A laminated dielectric filter is formed by strip line resonators on a plurality of dielectric sheets with different thicknesses.

Description: This invention relates to a laminated dielectric filter used mainly for dielectric antenna duplexers in high frequency radio devices such as mobile telephones. An antenna duplexer is a device for sharing one antenna by a transmitter and a receiver, and it is composed of a transmission filter and a reception filter. The invention is particularly directed to a laminated dielectric filter having a laminate structure by laminating a dielectric sheet and an electrode layer and baking into one body. Along with the advancement of mobile communications, recently, the antenna duplexer is used widely in many hand-held telephones and car-mounted telephones. An example of a conventional antenna duplexer is described below with reference to a drawing. Fig. 9 is a perspective exploded view of a conventional antenna duplexer. In Fig. 9, reference numerals 701 to 706 are dielectric coaxial resonators, 707 is a coupling substrate, 708 is a metallic case, 709 is a metallic cover, 710 to 712 are series capacitors, 713 and 714 are inductors, 715 to 718 are coupling capacitors, 721 to 726 are coupling pins, 731 is a transmission terminal, 732 is an antenna terminal, 733 is a reception terminal, and 741 to 747 are electrode patterns formed on the coupling substrate 707. The dielectric coaxial resonators 701, 702, 703, series capacitors 710, 711, 712, and inductors 713, 714 are combined to form a transmission band elimination filter. The dielectric coaxial resonators 704, 705, 706, and coupling capacitors 715, 716, 717, 718 compose a reception band pass filter. One end of the transmission filter is connected to a transmission terminal which is electrically connected with a transmitter, and the other end of the transmission filter is connected to one end of a reception filter, and is also connected to an antenna terminal electrically connected to the antenna. The other end of the reception filter is connected to a reception terminal which is electrically connected to a receiver. The operation of an antenna duplexer is described below. First of all, the transmission band elimination filter shows a small insertion loss to the transmission signal in the transmission frequency band, and can transmit the transmission signal from the transmission terminal to the antenna terminal while hardly attenuating it. By contrast, it shows a larger insertion loss to the reception signal in the reception frequency band, and reflects almost all input signal in the reception frequency band, and therefore the reception signal entering from the antenna terminal returns to the reception band pass filter. On the other hand, the reception band filter shows a small insertion loss to the reception signal in the reception frequency band, and transmits the reception signal from the antenna terminal to the reception terminal while hardly attenuating it. The transmission signal in the transmission frequency band shows a large insertion loss, and reflects almost all input signal in the transmission frequency band, so that the transmission signals coming from the transmission filter is sent out to the antenna terminal. In this design, however, in manufacturing dielectric coaxial resonators, there is a limitation in fine processing of ceramics, and hence it is hard to reduce its size. Downsizing is also difficult because many parts are used such as capacitors and inductors, and another problem is the difficulty in lowering the assembling cost. The dielectric filter is a constituent element of the antenna duplexer, and is also used widely as an independent filter in mobile telephones and radio devices, and there is a demand that they be smaller in size and higher in performance. Referring now to a different drawing, an example of a conventional block type dielectric filter possessing a different constitution from the above described structure is described below. Fig. 10 is a perspective oblique view of a block type dielectric filter of the prior art. In Fig. 10, reference numeral 1200 is a dielectric block, 1201 to 1204 are penetration holes, and 1211 to 1214, and 1221, 1222, 1230 are electrodes. The dielectric block 1200 is entirely covered with electrodes, including the surface of the penetration holes 1201 to 1204, except for peripheral parts of the electrodes on the surface of which the electrodes 1221, 1222 and others are formed. The operation of the thus constituted dielectric filter is described below. The surface electrodes in the penetration holes 1201 to 1204 serve as the resonator, and the electrode 1230 serves as the shield electrode. The electrodes 1211 to 1214 are to lower the resonance frequency of the resonator composed of the electrodes in the penetration holes, and functions as the loading capacity electrode. By nature, a 1/4 wavelength front end short-circuit transmission line is not coupled at the resonance frequency and shows a band stop characteristic, but by thus lowering the resonance frequency, an electromagnetic field coupling between transmission lines occurs in the filter passing band, so that a band pass filter is created. The electrodes 1221, 1222 are input and output coupling capacity electrodes, and input and output coupling is effected by the capacity between these electrodes and the resonator, and the loading capacity electrode. The operating principle of this filter is a modified version of a comb-line filter disclosed in the literature (for example, G.L. Matthaei, "Comb-Line Band-pass Filters of Narrow or Moderate Bandwidth"; the Microwave Journal, August 1963). The block type filter in this design is a comb-line filter composed of a dielectric ceramic (for example, see U. S. Patent 4,431,977). The comb-line filter always requires a loading capacity for lowering the resonance frequency in order to realize the band pass characteristic. Fig. 11 shows the transmission characteristic of the comb-line type dielectric filter in the prior art. The transmission characteristic shows the Chebyshev characteristic increasing steadily as the attenuation outside the bandwidth departs from the center frequency. In this construction, however, it is not possible to realize the elliptical function characteristic possessing the attenuation pole near the bandwidth of the transmission characteristic, and hence the range of selection is not sufficient for filter performance. Also, in such dielectric filter, for smaller and thinner constitution, the flat type laminate dielectric filter that can be made thinner than the coaxial type is expected henceforth, and several attempts have been made to design such a device. A conventional example of a laminated dielectric filter is described below. The following explanation relates to a laminated "LC filter" (trade mark) that is put into practical use as a laminated dielectric filter by forming lumped element type capacitors and inductors in a laminate structure. Fig. 12 is a perspective exploded view showing the structure of a conventional laminate "LC filter". In Fig. 12, reference numerals 1 and 2 are thick dielectric layers. On a dielectric sheet 3 are formed inductor electrodes 3a, 3b, and capacitor electrodes 4a, 4b are formed on a dielectric sheet 4, capacitor electrodes 5a, 5b on a dielectric sheet 5, and shield electrodes 7a, 7b on a dielectric sheet 7. By stacking up all these dielectric layers and dielectric sheets together with a dielectric sheet 6 for protecting the electrodes, an entirely laminated structure is formed. The operation of the thus constituted dielectric filter is described below. First, the confronting capacitor electrodes 4a and 5a, and 4b and 5b respectively compose parallel plate capacitors. Each parallel plate capacitor functions as a resonance circuit as connected in series to the inductor electrodes 3a, 3b through side electrodes 8a, 8b. Two inductors are coupled magnetically. The side electrode 8b is a grounding electrode, and the side electrode 8c is connected to terminals 3c, 3d connected to the inductor electrode to compose a band pass filter as input and output terminals (for example, Japanese Laid-open Patent No. 3-72706(1991)). In such a constitution, however, when the inductor electrodes are brought closer to each other to narrow the interval in order to reduce in its size, the magnetic field coupling between the resonators becomes too large, and it is hard to realize a favorable band pass characteristic narrow in the bandwidth. It is moreover difficult to heighten the unloaded Q value of the inductor electrodes, and hence the filter insertion loss is large. Another different conventional example of a laminated dielectric filter is described below with reference to an accompanying drawing. Fig. 13(a) and (b) shows the structure of a conventional laminated dielectric filter. In Fig. 13(a) and (b), 1/4 wavelength strip lines 820, 821 are formed on a dielectric substrate 819. Input and output electrodes 823, 824 are formed on the same plane as the strip lines 820, 821. The strip line 820 is composed of a first portion 820a (L 1 indicates the length of 820a) having a first line width W 1 (Z 1 indicates the characteristic impedance of W 1 ) confronting the input and output electrodes 823, a second portion 820b (L 2 indicates the length of 820b) having a second line width narrower than the first line width W 1 , and a third portion 820c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 (Z 2 indicates the characteristic impedance of W 2 ). Similarly, the strip line 821 is composed of a first portion 821a having a first line width W 1 confronting the input and output electrodes 824, a second portion 821b having a second line width narrower than the first line width W 1 , and a third portion 821c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 . The strip lines 820, 821 are connected with a short-circuit electrode 822, and the resonator 801b is in a pi-shape. A dielectric substrate 819 is covered by grounding electrodes 825, 826 at both surfaces. At one side 819a, side electrodes 827,828 are formed, and the grounding electrodes 825, 826, and short-circuit electrodes 822 are connected. On the other side 819b, side electrodes to be connected with the input and output electrodes 823, 824 respectively are formed. The strip lines 820, 821 are capacitively coupled with the input and output electrodes 823, 824, respectively, thereby constituting a filter as described for example, in U. S. Patent 5,248,949. In such constitution, however, same as the conventional block type dielectric filter, the elliptical function characteristic possessing the attenuation pole near the passing band of the transmission characteristic cannot be realized, and hence the scope of performance of the filter is not wide enough. US A-4,701,727 discloses a stripline tapped-line hairpin filter including a first substrate upon which a plurality of N hairpin resonators are disposed alternately on opposite surfaces of the first substrate. Each one of the hairpin resonators is in a parallel coupled relationship with an adjacent hairpin resonator disposed on an opposite surface of the first substrate. The first and last hairpin resonators each have an interconnected member disposed on the substrate for respectively coupling a signal into and out of the plurality of N hairpin resonators. Second and third substrates are included with each being respectively located adjacent to ones of the plurality of N hairpin resonators on opposite surfaces of the first substrate. First and second groundplanes are included with each groundplane respectively located adjacent the second and third substrates. It is a primary object of the invention to provide a laminated dielectric filter at low cost which has an excellent band pass characteristic with small insection loss and high bandwidth selectivity. Another object is to provide a laminate dielectric filter having a small and thin flat structure. The objects are achieved by the features of the claims. A first aspect of the invention provides a laminated dielectric filter where a first strip line resonator disposed on a first shield electrode through a first dielectric sheet with thickness t 1 , disposing second to n-th strip line resonators on the first strip line resonator through second to n-th dielectric sheets with thickness t 2 to t n (n being the number of strip line resonators, that is, 2 or more), disposing a second shield electrode on the n-th strip line resonator through the (n+1)-th dielectric sheet with thickness t n+1 , and setting thicknesses t 2 to t n different from thickness t 1 or t n+1 . In the laminated dielectric filter of the first aspect, a large coupling degree between resonators and a high unloaded Q-value are obtained, thereby realizing a small-sized filter having excellent filter characteristics such as low loss and high selectivity, and not requiring a wide floor area if formed in multiple stages. It is preferable that the maximum value of thicknesses t 2 to t n is set smaller than thickness t 1 or t n+1 . It is preferable that the maximum value of thicknesses t 2 to t n is set smaller than the maximum value of thicknesses t 1 and t n+1 . It is also preferable that the maximum value of thicknesses t 2 to t n is set smaller than either thickness t 1 or t n+1 . Additionally, it is preferable that the number n of strip line resonators is 3 or more (it is well-known to the skilled person that the number n can be 3 or more), and the thickness is equal in all from t 2 to t n . In the laminated dielectric filter of this, a large coupling degree between resonators and a high unloaded Q-value are obtained, thereby realizing a small-sized filter having excellent filter characteristics such as low loss and high selectivity, and not requiring a wide floor area if formed in multiple stages. It is preferable that the first shield electrode and second shield electrode are formed of inner layer electrodes enclosed by dielectric sheets. The shield electrode can be formed at the same process step as the strip line resonator electrode and capacity electrode, and hence manufacturing is easier. It is preferable that the first dielectric sheet and the (n+1)-th dielectric sheet are formed by laminating a plurality of thin dielectric sheets. By forming the thick dielectric sheet with thin dielectric sheets of standardized thickness, the manufacturing cost can be further reduced. It is preferable that the input and output coupling capacity electrode is each formed respectively in one of the thin dielectric sheets for composing the first dielectric sheet, and in one of the thin dielectric sheets for composing the (n+1)-th dielectric sheet. The filter can be smaller in size than in the magnetic field coupling system, by coupling the strip line resonator and input and output terminal by capacitive coupling. The calculation of the coupling amount is easy, and the input and output coupling amount can be adjusted by only varying the area of the electrode pattern, so that it is easy to design. It is preferable that the position of the center line of the first to n-th strip line resonators is shifted parallel in the lateral direction in every one of the first to n-th dielectric sheets. In the laminated dielectric filter of this embodiment, the coupling amount between the strip line resonators can be adjusted very easily. Furthermore, it is preferable that the first to n-th strip line resonators are used as front end short-circuit strip line resonators, and are laminated by aligning the direction of the short-circuit ends. Thus, the laminated dielectric filter is easy to design, and a small-sized filter can be attained. In addition,it is preferable that the broad grounding electrodes are formed at the short-circuit end side of the first to n-th strip line resonators, grounding side shield electrodes are formed of outer electrodes on the side of the short-circuit end side of the strip line resonator of the dielectric sheet composed of the first to (n+1)-th dielectric sheets, and the short-circuit end of the strip line resonator is connected and grounded to the grounding side shield electrode through the grounding electrode. In the laminated dielectric filter of the invention as set forth in this embodiment, a change in length of the broad grounding electrodes has a smaller effect on the resonance frequency than a change in length of the strip line resonator electrode, thereby suppressing the fluctuations of the resonance frequency due to variations from cutting the dielectric sheet. In addition, since the side is shielded by the side electrode of the grounding end grounding terminal, the field characteristic is hardly effected by external effects. It is preferable that the input and output coupling capacity electrode is each formed respectively in one of the thin dielectric sheets of the first dielectric sheet, and in one of the thin dielectric sheets of the (n+1)-th dielectric sheet, the take-out direction of the input and output coupling capacity electrode is the right side direction of the strip line resonator in one, and the left side direction of the strip line resonator in the other, and they are connected as input and output terminals to the side input and output electrodes formed of outer electrodes, provided at the right and left sides of the laminate composed of the first to (n+1)-th dielectric sheets. The take-out direction of the input and output terminal is set in the right side direction and left side direction of the strip line, and the input and output terminals can be isolated. Furthermore, it is preferable that the side shield electrodes are formed of outer electrodes at the sides of the laminate composed of the first to (n+1)-th dielectric sheets. It is preferable that the open side shield electrode is formed of outer electrode at the side of the open end side of the strip line resonator of the laminate composed of the first to (n+1)-th dielectric sheets. In the laminated dielectric filter of this embodiment, a change in filter characteristic by external effects can be prevented by the shield effect, and moreover the resonance of the shield electrode is suppressed to prevent deterioration of the filter characteristic. It is preferable that the line width at the short-circuit end side of the first to n-th strip line resonators is narrower than the line width of the open end side. In the laminated dielectric filter of the invention, the strip line has a wide part and a narrow part to compose the SIR structure, and therefore the length of the resonator is shorter than 1/4 wavelength, so that the filter can be reduced in size. It is also preferable that the line distance of the short-circuit end side narrow parts of the first to n-th strip line resonators is different from the line distance of the open end side broad parts. It is preferable that the positions of the line center lines of the open end side broad parts of the first to n-th strip line resonators are aligned vertically, and the positions of the line center lines of the short-circuit end side narrow parts are shifted parallelly in the lateral direction in every one of the first to n-th dielectric sheets. In the laminated dielectric filter of this vention, the electromagnetic coupling amount of wide parts and the electromagnetic coupling amount of narrow parts of the strip line can be independently set, and hence it is possible to design the attenuation pole at a desired frequency. By arranging up and down the positions of the line center lines of the wide parts of the strip line, the maximum coupling amount can be realized in the wide parts. Furthermore, the lateral width of the filter can be set at the smallest distance. It is preferable that the line width of the short-circuit end side of the first to n-th strip line resonators is set broader than the line width of the open end side. It is preferable that the line distance of the short-circuit end side broad parts of the first to n-th strip line resonators is different from the line distance of the open end side narrow parts. It is also preferable that the positions of the line center lines of the short-circuit end side broad parts of the first to n-th strip line resonators are aligned vertically, and the positions of the line center lines of the open end side narrow parts are shifted parallelly in the lateral direction in every one of the first to n-th dielectric sheets. In the laminated dielectric filter of the invention as set forth in this embodiment, the resistance loss of the high frequency current can be decreased by widening the grounding end side of the strip line resonator, so that the unloaded Q value can be improved. Furthermore, by arranging up and down the positions of the line center lines of the wide parts of the strip line, the maximum coupling amount can be realized in the wide parts. In addition, the lateral width of the filter can be set at the smallest distance. Fig. 1 is a perspective exploded view of a laminated dielectric filter in an first embodiment of the invention. Fig. 2 is a sectional view of section A-A' of the laminated dielectric filter in the first embodiment of the invention in Fig. 1. Fig. 3 is a perspective exploded view of a laminated dielectric filter in a second embodiment of the invention. Fig. 4 (a) is a sectional view of section A-A' of the laminated dielectric filter in the second embodiment of the invention in Fig. 3, and Fig. 4 (b) is a sectional view of section B-B'. Fig. 5 is a perspective exploded view of a laminated dielectric filter in a third embodiment of the invention. Fig. 6 (a) is a sectional view of section A-A' of the laminated dielectric filter in the third embodiment of the invention in Fig. 5, and Fig. 6 (b) is a sectional view of section B-B'. Fig. 7 is a perspective exploded view of a laminated dielectric filter in a fourth embodiment of the invention. Fig. 8 (a) is a sectional view of section A-A' of the laminated dielectric filter in the fourth embodiment of the invention in Fig. 7, and Fig. 8 (b) is a sectional view of section B-B'. Fig. 9 is a perspective exploded view of a dielectric antenna duplexer of the prior art. Fig. 10 is a perspective view of a block dielectric filter of the prior art. Fig. 11 is a graph showing transmission characteristic and reflection characteristic of a comb-line dielectric filter of the prior art. Fig. 12 is a perspective exploded view of a laminated LC filter of the prior art. Fig. 13 (a) and (b) is a perspective view of a laminated dielectric filter of the prior art. Example 1 A laminated dielectric filter in a first embodiment of the invention is described below by reference to drawings. Fig. 1 is a perspective exploded view of the laminated dielectric filter in the first embodiment of the invention. Fig. 2 is a sectional view of section A-A' in Fig. 1. In Fig. 1. dielectric sheets 310a, 310b, 310c, 310d, 310e, 310f, 310g, 310h are made of low temperature baking dielectric ceramics, and as dielectric materials, for example, Bi-Ca-Nb-O ceramics with the dielectric constant of 58 and other ceramic materials that can be baked at 950 degrees C or less are used, and green sheets are formed. The inner electrodes for composing the strip line resonator electrodes 311a, 311b, 311c, input and output coupling capacity electrodes 313a, 313b, and loading capacity electrodes 314a, 314b are laminated with dielectric sheets and baked integrally, while printing with electrode patterns with metal paste of high electric conductivity such as silver, copper and gold. The outer electrodes of the shield electrodes 315a, 315b, side electrodes 316a, 316b, and 317a, 317b, 317c, 317d are baked later with metal paste in this embodiment. The thicknesses t 2 , t 3 , ..., t n (n is the number of strip line resonators) of the dielectric sheet between the strip line resonator electrode layers, that is, the combined thickness of the dielectric sheets 310c and 310d, or the combined thickness of the dielectric sheets 310e and 310f is set differently from the thicknesses t 1 , t n+1 of the dielectric sheets between the strip line resonator electrode layer and shield electrode layer, that is, the combined thickness of the dielectric sheets 310a and 310b, or the combined thickness of the dielectric sheets 310g and 310h, and thereby a large coupling amount can be used without lowering the unloaded Q value of the resonator. More specifically, the maximum value of the thicknesses t 2 to t n is set smaller than either thickness t 1 or t n+1 , and preferably the total of thicknesses t 2 to t n is set smaller than either thickness t 1 or t n+1 . Moreover, when the number of strip line resonators is three or more, by equalizing all of thicknesses t 2 to t n , the thickness of the dielectric sheet can be standardized to a specific value, so that the manufacturing cost can be lowered. Furthermore, by forming the thick dielectric sheets 310a, 310h by laminating a plurality of thin dielectric sheets, all dielectric sheets can be formed of standardized same thin dielectric sheets, so that the manufacturing cost be further lowered. The strip line resonator electrodes 311a, 311b , 311c are connected and grounded to the side electrode 317d of the grounding end grounding element through the grounding electrodes 312a, 312b, 312c at one end. The change in length of the broad grounding electrodes has a smaller effect on the resonance frequency, as compared with the change in length of the strip line resonator electrode, and therefore fluctuations of the resonance frequency due to variations in the precision of cutting off the dielectric sheet can be suppressed. Moreover, the side electrode 317d of the grounding end grounding terminal acts also as the shield electrode of the grounding side for shielding the side, the filter characteristic is hardly affected from outside. In the embodiment, since the resonator is in laminate structure by aligning the direction of the short-circuit end, as the quarter wavelength end short-circuit type strip line resonator, it is therefore easy to design the same as in the comb-line filter, and a small-sized filter can be realized. The parallel flat plate capacitor composed between the input and output coupling capacity electrode 313a and strip line resonator electrode 311a, and the parallel flat plate capacitor composed between the input and output coupling capacity electrode 313b and strip line resonator electrode 311c both function as input and output coupling capacitors. The individual input and output coupling capacity electrodes 313a, 313b are connected to the input and output terminals 316a, 316b formed of the side electrodes. By coupling the strip line resonator and input and output terminals in capacity coupling system, the filter can be reduced in size in the magnetic field coupling system. In the capacity coupling system, calculation of coupling amount is easy, and the input and output coupling amount can be adjusted only by varying the electrode pattern area, so that it is easy to design. By setting the take-out direction of the input and output terminals 316a, 316b in the right side direction of the strip line in one and in the left side direction of the strip line in the other, the input and output terminals can be isolated. The parallel flat plate capacitor composed between the loading capacity electrodes 314a, 314b, and strip line resonator electrodes 311a, 311b, 311c function as the parallel loading capacitor for lowering the resonance frequency of the strip line resonator. Therefore, the length of the strip line resonators 311a, 311b, 311c can be set shorter than the quarter wavelength, thereby making it possible to operate a comb-line filter. In the region of the input and output coupling capacity electrodes 313a, 313b and the loading capacity electrodes 314a, 314b overlapping with the outer edge of the strip line resonator electrodes 311a, 311b, 311c, a dent is formed in the input and output coupling capacity electrodes and loading capacity electrodes, and the width of the electrodes is narrowed. By forming a narrow dent region, the change in the area of the overlapping region when position deviation of the strip line resonator electrode layer and capacity electrode layer can be set smaller as compared with the case without a dent. Since the entire filter is shielded by the upper and lower shield electrodes 315a, 315b formed of the outer electrodes, change of filter characteristic by the external effects can be prevented. The shield electrode is connected and grounded at the side electrodes 317a, 317b of the side grounding terminal, and the side electrode 317c of the grounding terminal at the open end, aside from the side electrode 317d of the grounding terminal at the grounding end side. By grounding the side electrode as the grounding terminal, at the open end, grounding side, and side surface of the strip line resonator, the resonance of shield electrode is suppressed, thereby preventing deterioration of the filter characteristic. Since the side electrodes 317a, 317b of the side grounding terminal function as side shield electrodes, the same as the side electrodes 317c, 317d, they have a shield effect to prevent the filter characteristic from being influenced by external effects. The open end capacity generated between the side electrode 317c of the open end side grounding terminal and the strip line resonator electrodes 311a, 311b, 311c is inserted parallel to the loading capacity, and hence the line length of the strip line resonator can be further shortened. Operation of the thus constituted laminated dielectric filter, the operation is described below. The electric operating principle of the filter in the embodiment is nearly same as the comb-line filter. The operating principle of the comb-line filter is disclosed in the cited literature (G.L. Matthaei, "Comb-Line Band-pass Filters of Narrow or Moderate Bandwidth"; the Microwave Journal, August 1963). First, the strip line resonator electrodes 311a, 311b, 311c are arranged by aligning in the direction of the grounding end, and by mutually coupling in the electromagnetic field, they operate a comb-line filter. The electromagnetic field coupling amount among the strip lines is adjusted by shifting the position of the center line of the strip line in every laminate sheet laminated up and down. Therefore, the adjustment of the coupling amount is very easy. The coupling amount is the largest when the positions of the center lines of the strip lines are matched. In the conventional invention of arranging the strip lines laterally on a same plane, the gap between lines is about 200 µm at minimum due to limitations of the printing precision, and there was a limitation in the magnitude of the coupling amount. However, in the embodiment of overlapping the strip lines up and down in the innovation, the thickness of the dielectric sheets 310d, 310f between the strip lines may be set as thin as 20 µm, so that a very large coupling amount may be realized. In addition, since the two strip line resonator electrodes contact over a wide area, the coupling amount is further increased. Since the electromagnetic field coupling between the strip lines is zero at a frequency corresponding to one quarter of the wavelength, the band pass filter cannot be composed in this state, but by shifting the resonance frequency by the loading capacity composed of the loading capacity electrodes 314a, 314b, and strip line resonator electrodes 311a, 311b, 311c, the required interstage coupling amount is obtained. In this embodiment, incidentally, by forming a capacity in both upper and lower directions of one loading capacity electrode, the number of loading capacity electrode layers is decreased, so that it is easy to manufacture. The input and output coupling is effected by electric field coupling of the input and output terminals and strip lines by the input and output coupling capacity electrodes 313a, 313b. The input and output coupling capacity forms a part of the admittance inverter. The capacity coupling embodiment is advantageous because it can be realized easily in a small size since the coupling embodiment of the band pass filters a relatively narrow band. Furthermore, in the embodiment of arranging the strip lines in the lateral direction, since the high frequency current is concentrated in the edge of the line, and the unloaded Q is lowered. However, in the embodiment of overlapping the strip lines up and down of the invention, the high frequency current is distributed relatively uniformly over the entire width of the line, so that a high unloaded Q value is realized. Hence, the insertion loss of the filter can be reduced. Thus, according to the invention, possessing a filter characteristic of low loss, a planar laminated dielectric filter of small size and thin thickness can be realized. Example 2 A laminated dielectric filter in a second embodiment of the invention is described by reference to the drawings. Fig. 3 is a perspective exploded view of the laminated dielectric filter in the twelfth embodiment of the invention. Fig. 4 (a) is a sectional view of section A-A' in Fig. 3, and Fig. 4 (b) is a sectional view of section B-B'. In Fig. 3, reference numerals 330a, 330b, 330c, 330d, 330e, 330f, 330g, 330h indicate dielectric sheets. Reference numerals 331a, 331b, 331c are strip line resonator electrodes, 335a, 335b are input and output coupling capacity electrodes, and 336a, 336b indicate shield electrodes, being formed of inner electrodes laminated on the dielectric sheets. In the second embodiment, which is different from the first embodiment, the shield electrodes are formed of inner electrodes. In this embodiment, the shield electrodes can be formed in the same embodiment as in strip line resonator electrodes and capacity electrodes, and are hence easy to manufacture. Since the entire filter is shielded by the upper and lower shield electrodes 336a, 336b formed of inner electrodes, thereby preventing the filter characteristic from changing due to external effects same as in the first embodiment. Side electrodes 337a, 337b as input and output terminals, and side electrodes 338a, 338b, 338c, 338d are formed of external electrodes baked after applying metal paste. Aside from the side electrode 338d of the grounding terminal at the grounding end side, the shield electrodes are connected and grounded to the side electrodes 338a,338b of the side grounding terminals and the side electrode 338c of the grounding terminal of the open end side. By grounding the side electrodes which become grounding terminals, at both open end and grounding end sides of the strip line resonator, resonance of the shield electrode is suppressed, and deterioration of filter characteristic is prevented. The strip line resonator electrodes 331a, 331b, 331c consist of grounding end side narrow parts 333a, 333b, 333c narrowed in the line width at the grounding end side, and open end side broad parts 332a, 332b, 332c broadened in the line width at the open end side. The grounding ends of the strip line resonator electrodes 331a, 331b, 331c are connected and grounded to the side electrode 338d of the grounding end side grounding terminal through the grounding electrodes 334a, 334b, 334c. A parallel flat plate capacitor composed between the input and output coupling capacity electrode 335a and strip line resonator electrode 331a, and a parallel flat plate capacitor composed between the input and output coupling capacity electrode 335b and strip line resonator electrode 331c both function as input and output coupling capacitors. The input and output coupling capacity electrodes 335a, 335b are connected to input and output terminals 337a, 337b formed of side electrodes. In this embodiment, as in the first embodiment, the thicknesses t 2 , t 3 , ..., t n (n is the number of strip line resonators) of the dielectric sheets between the strip line resonator electrode layers, or the thicknesses of the dielectric sheets 330d, 330e are set smaller than the thicknesses t 1 , t n+1 of the dielectric sheets between the strip line resonator electrode layer and shield electrode layer, that is, the total thickness of the dielectric sheets 330b and 330c, or the total thickness of the dielectric sheets 330f and 330g, so that a great coupling amount is obtained without lowering the unloaded Q value of the resonator. For example, in one production, the thickness of dielectric sheets 330b, 330g is 500 µm, the thickness of dielectric sheets 330c, 330f is 55 µm, and the thickness of dielectric sheets 330d, 330e is 44 µm, and a favorable filter characteristic could be obtained at this time. That is, supposing the maximum value of thicknesses t 2 , t 3 , .. ., t n to be t max , it is desired that t max be smaller than either t 1 or t n+1 . More preferably, the total of thicknesses t 2 , t 3 , ..., t n should be smaller than the total of t 1 and t n+1 . Further preferably, the total of thicknesses t 2 , t 3 , ...,tn should be smaller than either thickness t 1 or t n+1 . In such conditions, the coupling degree necessary for filter design and the high unloaded Q value could be obtained at the same time. Moreover, by forming thick dielectric sheets 330b, 330g by laminating a plurality of thin dielectric sheets, and equalizing the thickness of all dielectric sheets 330d, 330e between strip line resonators, all dielectric sheets can be formed by thin dielectric sheets of standardized thickness, so that the manufacturing cost can be reduced. Operation of the thus constituted laminated dielectric filter, the operation is described below. The electric operating principle of the filter in this embodiment is slightly different from the principle of the filter in the first embodiment. That is, in the first embodiment, the operating principle is basically the comb-line filter. In the second embodiment, however, by using the SIR (stepped impedance resonator) structure instead of loading capacity, the electromagnetic field coupling amounts of the first transmission lines and second transmission lines are set independently, and a passing band and an attenuation pole are generated in the transmission characteristic. This basic constitution is the same as in the laminated dielectric filter of the first embodiment. First, the strip line resonator electrodes 331a, 331b, 331c are arranged by aligning the direction of the grounding ends, and the open end side broad parts 332a, 332b, 332c and the grounding end side narrow parts 333a, 333b, 333c are respectively coupled electromagnetically. Each strip line constitutes the SIR structure with the broad parts and narrow parts. Therefore, the length of the strip line resonators 331a, 331b, 331c can be shorter than the quarter wavelength. The electromagnetic field coupling amount between the strip lines is adjusted by shifting the position of the strip line in the vertical direction. By deviating the line center line of the broad parts and narrow parts of the strip lines from the same line, the electromagnetic field coupling amount of the broad parts and the electromagnetic field coupling amount of the narrow parts of the strip lines can be set independently. By independently setting the coupling amounts in this way only, it is possible to design to form an attenuation pole at a desired frequency. This operating principle has been explained in the filter of the first embodiment. By setting all at the same position, with the dielectric sheets laminating vertically the line center lines of the broad parts of the strip lines, the maximum coupling amount can be realized in the broad parts. Furthermore, since the vertical positions of the electrodes are aligned, the filter width can be minimized, so that the filter size can be reduced. On the other hand, the coupling amount of the narrow parts can be adjusted by shifting the position of the line center line by every dielectric sheet. In this way, by electromagnetic field coupling of the open end side broad parts and grounding end side narrow parts, independently, not only the band pass characteristic is shown in the passing band, but also an attenuation pole can be formed at a desired frequency of transmission characteristic. Therefore, a selectivity characteristic superior to the Chebyshev characteristic can be realized. Thus, according to the embodiment, the same effects as in the first embodiment can be achieved, and an attenuation pole can be formed at a desired frequency of transmission characteristic, and excellent selectivity characteristic is achieved. Thus a filter characteristic of small size and low loss is achieved. Example 3 A laminated dielectric filter in a third embodiment of the invention is described below by referring to the accompanying drawings. Fig. 5 is a perspective exploded view of the laminated dielectric filter in the third embodiment of the invention. Fig. 6 (a) is a sectional view of section A-A' in Fig. 5, and Fig. 6 (b) is a sectional view of section B-B'. In Fig. 5, reference numerals 350a, 350b, 350c, 350d, 350e, 350f, 350g, 350h, 350i, 350j indicate dielectric sheets. Reference numerals 351a, 351b, 351c are strip line resonator electrodes, 353a, 353b are input and output coupling capacity electrodes, 354a, 354b are shield electrodes, and 355a, 355b are coupling shield electrodes, which are formed of inner electrodes laminated on the dielectric sheets. Side electrodes 357a, 357b as input and output terminals, and side electrodes 358a, 358b, 358c, 358d as grounding terminals are formed of outer electrodes baked after application of metal paste. The shield electrodes are connected and grounded to the side electrodes 358a, 358b of the side grounding terminals and side electrode 385c of grounding terminal of open end side, aside from the side electrode 358d of grounding terminal at grounding end side. The grounding ends of strip line resonator electrodes 351a, 351b, 351c are connected and grounded to the side electrode 358d of the grounding terminal at the grounding end side through grounding electrodes 352a, 352b, 352c. A parallel flat plate capacitor composed between the input and output coupling capacity electrode 353a and strip line resonator electrode 351a, and a parallel flat plate capacitor composed between the input and output coupling capacitor composed between the input and output coupling capacity electrode 353b and strip line resonator electrode 351c both function as input and output coupling capacitors. The input and output coupling capacity electrodes 353a, 353b are connected to input and output terminals 357a, 357b formed of side electrodes. In the third embodiment, different from the first and second embodiments, the coupling amount between the strip line resonators is controlled the electric field coupling windows or the magnetic field coupling windows 356a, 356b formed in the coupling sheield electrodes 355a, 355b. Depending on the size, shape and position of the coupling window, it is easy to control from a large coupling amount to a small coupling amount, so that a filter characteristic in a broad range from wide band to narrow band is realized. By capacity coupling for input and output coupling, the design is easy, and the filter size can be reduced. Thus, according to the embodiment, aside from the effects of the first and second embodiments, a filter characteristic in a broad range from wide band to narrow band can be attained by a simple design. Example 4 A laminated dielectric filter in a fourth embodiment of the invention is described below while referring to the drawings. Fig. 7 is a perspective exploded view of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 8 (a) is a sectional view of section A-A' in Fig. 7, and Fig. 8 (b) is a sectional view of section B-B'. In Fig. 7, reference numerals 370a, 370b, 370c, 370d, 370e, 370f are dielectric sheets. Reference numerals 371a, 371b, 371c are strip line resonator electrodes, 375a, 375b are input and output coupling capacity electrodes, and 377a, 377b are shield electrodes, which are formed of inner electrodes laminated on dielectric sheets. Side electrodes 378a, 378b as input and output terminals, and side electrodes 379a, 379b, 379c, 379d as grounding terminals are formed of outer-electrodes by baking metal paste afterwards. Shield electrodes are connected and grounded to the side electrodes 379a, 379b of the side grounding terminals and the side electrode 379c of the grounding terminal at the open end side, aside from the side electrodes 379d of the grounding terminal at the grounding end side. The strip line resonator electrodes 371a, 371b, 371c consist of grounding end side broad parts 373a, 373b, 373c widened in the line width at the grounding end side, and open end side narrow parts 372a, 372b, 372c narrowed in the line width at the open end side. The grounding ends of the strip line resonator electrodes 371a, 371b, 371c are connected and grounded to the side electrode 379d of the grounding terminal at the grounding end side, through the grounding electrodes 374a, 374b, 374c. In the fourth embodiment, the broad parts come to the grounding end side of the strip line resonator, which is opposite to the constitution of the second embodiment. By shifting the line center lines of the grounding end side broad parts and line center lines of open end side narrow parts of each strip line, without aligning on the same line, in this embodiment, too, same as in the second embodiment, the electromagnetic field coupling amount of the broad parts and narrow parts of the strip line resonator can be controlled independently. Therefore, an attenuation pole can be formed at a desired frequency of transmission characteristic of the filter, and an excellent selectivity is obtained. Additionally, by forming broad parts at the grounding end side of the strip line resonator, the resistance loss of the high frequency current flowing in the strip line can be reduced, and hence the unloaded Q can be improved. Furthermore, by setting the line center lines of the broad parts of the strip lines all at the same position on the dielectric sheets laminated vertically, a maximum coupling amount can be realized in the broad parts. Since the vertical positions of the electrodes are aligned, the width of the filter can be minimized, so that the filter can be reduced in size. An inter-digital type capacitor 376a composed between the input and output coupling capacity electrode 375a and strip line resonator electrode 371a, and an inter-digital type capacitor 376b composed between the input and output coupling capacity electrode 375b and strip line resonator electrode 371c both function as input and output coupling capacitors. The input and output coupling capacity electrodes 375a, 375b are connected to input and output terminals 378a, 378b formed of side electrodes. By composing the input and output coupling capacity by interdigital type capacitor, a large coupling capacity is obtained, and a band pass filter characteristic of wide band is realized. Thus, according to the embodiment, aside from the same effects as in the first through third embodiments of obtaining a laminated dielectric filter of low loss, small size, and thin and flat structure, the number of dielectric sheets and the number of times of electrode printing can be decreased, and the manufacturing is easier.

Claims: A laminated dielectric filter formed by a first strip line resonator electrode (311a) stacked on a first shield electrode (315a) through a first dielectric sheet (310a,b) with a thickness t 1 , second to n-th strip line resonator electrodes (311b, 311c) stacked on the first strip line resonator electrode (311a) through second to n-th dielectric sheets (310c ... f) with thickness of t 2 to t n (n being a plural number of strip line resonator electrodes), a second shield electrode (315b) stacked on the n-th strip line resonator electrode (311c) through the (n+1)-th dielectric sheet (310h,g) with thickness t n+1 , with thicknesses t 2 to t n different from thickness t 1 or t n+1 , characterised in that the strip line resonator electrodes (311a, 311b, 311c) are stacked in vertical direction so as to be overlapped up and down. The laminated dielectric filter of claim 1, wherein the thicknesses t 2 to t n have a maximum value set smaller than thickness t 1 or t n-1 . The filter of claim 1 or 2, wherein the total of thicknesses t 2 to t n is set smaller than the total of thicknesses t 1 and t n+1 . The filter of claim 1, 2 or 3, wherein the total of thicknesses t 2 to t n is set smaller than either thickness t 1 or t n+1 . The filter of any of claims 1 to 4, wherein the number n of strip line resonators is 3 or more, and the thickness of t 2 to t n are equal. The filter of any of claims 1 to 5, wherein the first shield electrode and second shield electrode are formed of inner layer electrodes (336a, 336b, respectively) enclosed by dielectric sheets. The filter of any of claims 1 to 6, wherein the first dielectric sheet and the (n+1)-th dielectric sheet comprise a laminated plurality of thin dielectric sheets. The laminated dielectric filter of claim 7, wherein an input and output coupling capacity electrode (313a, 313b, respectively) is formed respectively in one of the thin dielectric sheets of the first dielectric sheets, and in one of the thin dielectric sheets of the (n+1)-th dielectric sheet. The filter of any of claims 1 to 8, wherein the position of the center line of the first to n-th strip line resonators is shifted parallel in the lateral direction in every one of the first to n-th dielectric sheets. The filter of any of claims 1 to 9, wherein the first to n-th strip line resonator electrodes (311a, 311b, 311c) are used as front end short-circuit strip line resonators, and are laminated by aligning the direction of the short-circuit ends, as a quarter wavelength one end short-circuit type strip line resonator. The laminated dielectric filter of claim 10, wherein broad grounding electrodes (331a, 331b, 331c) extending vertically to strip line resonator electrodes are formed at the short-circuit end side of the first to n-th strip line resonator electrodes (311a, 311b, 311c) grounding side shield electrodes are provided by outer electrodes (317d) on the side of the short-circuit end side of the strip line resonator of the dielectric composed of the first to (n+1)-th dielectric sheets, and the short-circuit end of the strip line resonator electrode is connected and grounded to the grounding side shield electrode (317d) through the grounding electrode. The filter of claim 10 or 11, wherein the input and output coupling capacity electrode (313a, 313b, respectively) is formed respectively in one of the thin dielectric sheets of the first dielectric sheet, and in one of the thin dielectric sheets of the (n+1)-th dielectric sheet, the input and output capacity coupling electrodes are located at the right side direction of the strip line resonator electrode in one, and at the left side of the strip line resonator electrode in the other, and they are connected as input and output terminals to the side input and output electrodes formed of outer electrodes, provided at the right and left sides of the laminate comprised by the first to (n+1)-th dielectric sheets. The filter of claim 10, 11 or 12, wherein side shield electrodes are formed of outer electrodes at the sides of the laminate-composed of the first to (n+1)-th dielectric sheets. The filter of any of claims 10 to 13, wherein an open side shield electrode is formed of outer electrode at the side of the open end side of the strip line resonator of the laminate composed of the first to (n+1)-th dielectric sheets. The filter of any of claims 10 to 14, wherein the line width at the short-circuit end side (333a, 333b, 333c) of the first to n-th strip line resonator electrodes is narrower than the line width of the open end side (332a, 332b, 332c). The laminated dielectric filter of claim 15, wherein the line distance of the short-circuit end side narrow parts (333a, 333b, 333c) of the first to n-th strip line resonators is different from the line distance of the open end side broad parts (332a, 332b, 332c). The filter of claim 15 or 16, wherein the positions of the line center lines of the open end side broad parts (332a, 332b, 332c) of the first to n-th strip line resonator electrodes are aligned vertically, and the positions of the line center lines of the short-circuit end side narrow parts are shifted parallel in the lateral direction in every one of the first to n-th dielectric sheets. A filter of any of claims 10 to 14,wherein the line width of the short-circuit end side (373a, 373b, 373c) of the first to n-th strip line resonator electrodes is set broader than the line width of the open end side (372a, 372b, 372c). The laminated dielectric filter of claim 18,wherein the line distance of the short-circuit end side broad part (373a, 373b, 373c) of the first to n-th strip line resonator electrodes is different from the line distance of the open end side narrow parts (372a, 372b, 372c). The filter of claim 18 or 19, wherein the positions of the line center lines of short-circuit end side broad parts (373a, 373b, 373c) of the first to n-th strip line resonator electrodes are aligned vertically, and the positions of the line center lines of the open end side narrow parts (372a, 372b, 372c) are shifted parallel in the lateral direction in every one of the first to n-th dielectric sheets.

Inventors: AIZAWA KIMIO, FUJINO TAKASHI, ISHIZAKI TOSHIO, KUSHITANI HIROSHI, NAKAKUBO HIDEAKI, NAKAMURA TOSHIAKI, SASAKI ATSUSHI, SATOH YUKI, AIZAWA, KIMIO, FUJINO, TAKASHI, ISHIZAKI, TOSHIO, KUSHITANI, HIROSHI, NAKAKUBO, HIDEAKI, NAKAMURA, TOSHIAKI, SASAKI, ATSUSHI, SATOH, YUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P1